The present invention relates to an amorphous photoelectric converting device in which a plurality of semiconductor thin films or elements of p-i-n structure are placed one on top of the other in the direction perpendicular to the bonded surface, such as in a multijunction solar cell.
Solar cells and photosensors are among the applications of a photovoltaic element composed of an amorphous semiconductor film, especially an amorphous silicon (a-Si:H) film formed through decomposition of a silane gas by glow discharge or ultraviolet light. Current research relating to solar cells is aimed at the improvement of their conversion efficiency and reliability, which is necessary for the cells to be put to practical use.
A multijunction solar cell is known to generate a high output voltage. It is made up of a-Si:H elements, each element having the p-i-n structure, as shown in FIG. 2. In FIG. 2 there are shown a transparent substrate 1, a transparent electrode layer 2; a first element composed of a first p-type layer 31, a first i-type layer 41, and a first n-type layer 51; a second element composed of a second p-type layer 32, a second i-type layer 42, and a second n-type layer 52, and a reverse metal electrode layer 6, which are placed one on top of the other. The adjacent first n-type layer 51 and second p-type layer 32 are both made of microcrystalline silicon (.mu.C-Si:H) which permits increased contact between the two layers. Additionally, a positive terminal 21 is formed on the exposed surface of the transparent electrode layer 2.
A solar cell of this type is improved in efficiency if the second i-type layer 42 is made of a material having a narrower optical band width than that for the first i-type layer 41. For example, a solar cell having a first i-type layer of a-Si:H and a second i-type layer of a-SiGe:H can utilize sunlight more efficiently. Further studies are being made on solar cells of multiple layer type formed by laminating together or more units of p-i-n structure.
Unfortunately, the solar cell of multiple layer type suffers from a disadvantage that when it is allowed to stand at high temperatures for a long time, it becomes seriously deteriorated in characteristic properties such as open circuit voltage, curve factor, and conversion efficiency. The deterioration is caused by the diffusion of impurities into the interface between the n-type semiconductor layer of one element and the p-type semiconductor layer of the other adjacent element. The diffused impurities impair the interfacial ohmic contact between the two adjacent layers.